Logging method and apparatus using a sonde equipped with measuring pads

ABSTRACT

Dip measurements and investigation of geological formations traversed by a deviated borehole is carried out by means of an apparatus comprising a measuring sonde and a cartridge connected to a cable. The sonde carries four measuring pads regularly distributed by means of articulated arms which keep the pads parallel to the axis of the sonde. The opposite arms are forced to stay symmetrical in relation to the sonde. 
     The sonde is connected to the cartridge so that its axis can depart angularly from the axis of the cartridge, enabling the latter to rest on the borehole wall. The pads have a length smaller than twice the transverse dimension D of the sonde.

BACKGROUND OF THE INVENTION

The invention relates to the investigation of formations traversed by aborehole by means of a sonde equipped with measuring pads.

To carry out certain measurements in boreholes, and in particular fordip measurements, pads are applied against the borehole walls, thesepads being equipped with electrodes or transducers carried by armsarticulated on a sonde body member and distributed symmetrically aroundthe latter. The sonde body member is connected to an upper part or acartridge which is suspended from a cable allowing electricaltransmission to surface equipment. Resilient means act on the arms so asto apply the pads against the walls with a suitable pressure, and asystem controlled from the surface equipment makes it possible toretract the arms along the sonde body member against the action of theresilient means. The pads are placed in contact with the walls onlyduring measurement movements.

U.S. Pat. No. 3,685,158 (J. Planche) describes a dipmeter apparatus withfour arms associated in two independent pairs, each pair comprising twoopposite arms whose movements are linked. The opposite pads are thusalways spread symmetrically in relation to the sonde body member, butthe pads of one pair can be spread differently in relation to the padsof the other pair. This arrangement allows the pads to remain appliedagainst the walls in the case of an oval hole, the pads then forming(view in plan) the apexes of a diamond in the center of which is locatedthe sonde body member.

Furthermore, in the above-mentioned apparatus, the pads are forced toremain coplanar so as to simplify the processing of measurement signals.In other words, the pads can move laterally in relation to the bodymember only in a plane perpendicular to the sonde axis. This isaccomplished by mounting the pads on slides carried by the arms andconnecting them to the sonde body member by linkage systems in the formof a Y.

In deviated boreholes, the weight of the apparatus has a radialcomponent with acts on the lower pad(s) against the action of theresilient means so that the upper pad(s) symmetrical in relation to thelower pad(s) have a tendency to leave the wall, thereby deterioratingthe corresponding measurement signals. To reduce the effect of boreholeinclination, it is possible, as provided for in U.S. Pat. No. 3,423,671(A. Vezin), to mount elastic centering devices at the center of gravityof the apparatus. However, the compensation obtained is only partial andthese centering devices, constantly in contact with the walls, undergorapid wear. They can, moreover, be used only in boreholes of sufficientdiameter.

U.S. Pat. No. 3,474,541 (W. E. Cubberly, Jr.) describes a caliperlogging apparatus adapted to deviated boreholes in which an articulatedjoint connects the upper part (connected to the cable) to the sonde. Thesonde has four arc springs arranged in two opposite pairs, the ends ofwhich are connected to the sonde while remaining mobile along thelongitudinal direction of the sonde. The movements of the ends of eachpair of springs are measured to determine the size of the hole along twoperpendicular directions. This known apparatus does not comprise padskept parallel to the axis of the sonde, and hence differs basically fromthe invention.

SUMMARY OF THE INVENTION

It is thus an object of the invention to improve the application of thepads on the borehole walls in the case of a deviated borehole.

Accordingly, the invention provides, firstly, the possibility of anangular offset between the axis of the sonde and the axis of thecartridge, so that the cartridge can rest on the borehole wall duringthe measurement movement. The weight of the cartridge hence is notinvolved in the radial component acting on the lower pads. Only theweight of the sonde intervenes, thereby substantially reducing thiscomponent.

Secondly, a pad length (dimension parallel to sonde axis) at mostsubstantially equal to twice the transverse dimension D of the sonde,and preferably substantially equal to 1.5 D, is provided. As the upperend of the sonde connected to the cartridge rests on the borehole wall,the axis of the sonde is consequently offset angularly from the axis ofthe borehole. A similar offset exists between the contact surfaces ofthe pads, kept parallel to the axis of the sonde, and the surface of theborehole walls. The resulting distance between the walls and themeasuring elements (electrodes or transducers) generally located at padmid-length is proportional to the length of the pads. With a length atmost substantially equal to 2D, D being the transverse dimension of thesonde, and preferably of the order of 1.5 D, the distance between themeasuring elements and the walls is minimized. Such a length correspondsmoreover to a sifficient application surface to avoid the sinking of thepads into the walls in the case of nonconsolidated formations.

It is desirable, furthermore, to be able to adjust the resilient forceapplied to the pads. An increase in the pad application pressure isrequired in particular, whatever the inclination of the borehole, in thepresence of very viscous drilling mud (gamboo). The U.S. Pat. Nos.3,423,671 and 3,685,158 mentioned above provide an adjustment in theloading force but the adjustment range is insufficient, so that in orderto obtain a very large force when required, it would be necessary to usevery powerful springs, giving an excessive force for normal conditions.It is therefore yet another object of the invention to provide a methodand apparatus for applying against the borehole walls the measuring padsof a logging sonde, characterized by the fact that said pads are placedat the end of articulated arms, and a first resilient force which issubstantially constant whatever the position of the arms is applied tothe pads as well as a second adjustable resilient force.

It is thus possible, in an independent manner, to define the firstsubstantially constant force so that it corresponds to the appropriatevalue for normal conditions, and also to define the second adjustableforce so that it exhibits an extensive adjustment range.

The invention will be better understood through the description givenbelow read in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic view of a dipmeter apparatus witharticulated arms according to the invention, during a measurement in adeviated borehole;

FIG. 2 shows in greater detail a pad in contact with the borehole wall;

FIG. 3 is a sectional view of the joint between the upper part and thelower part of the apparatus;

FIG. 4 is an axial sectional view showing in greater detail the lowerportion of the sonde and in particular the link with the arms;

FIG. 5 is a cross-sectional view along the plane V--V of FIG. 4 inlarger scale;

FIG. 6 is an axial sectional view of the portion of the sonde comprisingthe arm control elements;

FIG. 7 is a perspective view of certain parts represented in FIG. 4;

FIG. 8 is a diagram of the hydraulic system for actuating the arms;

FIG. 9 shows a detail of the diagram of FIG. 8.

DETAILED DESCRIPTION OF A REPRESENTATIVE EMBODIMENT

FIG. 1 represents an oilfield borehole B traversing geologicalformations. In the section in which the measurements are carried out,this borehole exhibits a large inclination in relation to the vertical.

The dipmeter apparatus, designated generally by item 10, is suspendedfrom a cable 11 connected to surface equipment, not shown. The cable 11allows electrical transmission between the surface equipment and theapparatus 10. The apparatus 10 is designed in the form of an elongatedbody member of circular section adapted to movement in the borehole. Itis composed of a lower part or sonde 12 and an upper part or cartridge13 connected by a joint 14.

The sonde 12 includes a body member 15 whose axis AA' defines alongitudinal direction, and four measuring pads placed at 90 degreesaround the body member, each carried by two arms. The arms include amain arm and a secondary arm hinged on the sonde body member 15. Forclarity, FIG. 1 shows only one pair of opposite pads 16, 17 carriedrespectively by the arms 18, 19 and 20, 21, the references 18 and 20designating the main arms and the references 19, 21 the secondary arms.Each pair of arms carrying a pad constitutes a parallelogram with thepad and the sonde body member 15 so that the pads are always parallel tothe axis of the sonde body member 15.

Elastic means including leaf springs 29 act on the arms to move themlaterally away from the sonde body member and to apply the pads againstthe borehole walls. The mechanism described below, mounted in the sondebody member and controlled from the surface equipment, makes it possibleto retract the arms against the sonde. The pads are placed in contactwith the walls only during measurement movements.

The main arms are coupled in opposite pairs so that two opposite arms,for example the arms 18 and 20, are always extended symmetrically inrelation to the sonde body member 15. However, the two pairs of mainarms are independent, i.e. the main arms of one pair can be extendeddifferently in relation to the main arms of the other pair. Thisarrangement allows the pads to remain in contact with the walls in thecase of an oval-shaped hole, the pads then forming (plan view) theapexes of a diamond whose center is the axis AA' of the sonde bodymember. It should however be noted that when the two pairs of main armshave different extensions in relation to the body member, the pairs ofpads connected respectively with them take on different longitudinalpositions. This arrangement of the pads is said to be "non-coplanar".

As seen in FIG. 2, each pad has an electrode system 25 which makes itpossible to determine the resistivity of the formation opposite the pad.An appropriate electrode system is described, for example, in U.S. Pat.No. 4,251,773 (Cailliau et al.). This system comprises two electrodesplaced side-by-side at the mid-length of the pad.

Means are provided in the apparatus to supply the needs of theelectrodes and to receive and shape the measurement signals receivedtherefrom. These means do not form part of the object of the inventionand are therefore not described here. The four sets of signals collectedare correlated in a known manner to determine the dip properties of theformations.

Furthermore, means sensitive to the lateral movement of each pair ofarms are mounted in the sonde body. From the signals obtained, it ispossible to calculate the size of the hole along two perpendiculardirections and thus determine the shape of this hole at the differentdepths. This also gives the longitudinal positions of the pads of eachpair in relation to the reference point on the body member. Thesepositions, as it was seen, are variable and must be known in order to beable to carry oug suitable depth corrections.

Moreover, the dip properties (inclination and orientation of the line ofgreatest slope relative to horizontal) of the formations must bedetermined not in relation to the axis of the borehole but in relationto a terrestral reference. For this purpose, the sonde includes meanssensitive to the inclination of the sonde axis in relation to thevertical and means sensitive to the orientation or azimuth of areference plane going through this axis in relation to a given directionsuch as magnetic north. These means are grouped in a unit 30 mounted inthe upper part of the sonde. This unit comprises three accelerometersfor inclination determination and three magnetometers for orientationdetermination. Such apparatus are known and shall not be described herein detail.

The cartridge 13 comprises two parts, an electronic cartridge 31connected to the joint 14 and a telemetering cartridge 32 connected tothe cable 11. The electronic cartridge 31, in connection with thesurface equipment, produces excitation and control signals and processesthe measurement signals. The telemetering cartridge 32 constitutes theinterface between the cable 11 and the electronic cartridge 31.

The joint 14 is shown in greater detail in FIG. 3. Its upper part 33,fixed to the cartridge, ends in a knuckle 34 in the shape of a truncatedsphere. The lower part 35 of the joint, fixed to the sonde, includes atits end a bore 36 with a spherical surface which cooperates with theknuckle 34. The spherical bore 36 is extended by a threaded bore 37 inwhich is screwed a ring 38 serving to retain the knuckle 34. The bore ofthe ring 38 includes a spherical portion 39 for contact with the knuckle34 and a conical portion 40 which allows an angular clearance betweenthe lower part 35 and the upper part 33. The ring 38 is made up of twohalves to allot it to be screwed after the introduction of the knuckle34 into the bore 36.

The knuckle 34 includes, on its outer surface, slots 41 in which areengaged respective pins 42 fixed to the lower part 35. These pinsprevent the lower part from rotating about its axis in relation to theupper part. However, the pins 42 do not penetrate fully into the slots,so that the lower part can move angularly in any direction.

The passage required for electrical conductors located in a duct 43 isprovided by a central hole 44 formed in the upper part 33 and a centralhole 45 extending the bore 36 in the lower part 35. Channels 46 allowthe drilling mud to penetrate into these holes 44, 45 while providingbalanced pressure on the different parts. The duct 43 is protected fromthe mud by a pressure-resistant flexible metal tube 47. The tube 47 hasits ends engaged in sealing elements 48, 49 respectively closing centralhole 44 of the upper part and the central hole 45 of the lower part. Toprovide effective prevention of mud passage, seals such as 50, 51 aremounted respectively on the outer surface and the inner surface of eachsealing element. It will be noted that in each sealing element the borereceiving the tube 47 has a terminal flare 52 allowing an angular offsetof the flexible tube 47 with respect to the considered sealing element.

The use of an articulated link between the sonde and the cartridge, isan inclined borehole, permits the cartridge to rest on the boreholewall, as shown in FIG. 1. The weight of the cartridge consequently isnot involved in the radial weight component which is applied on thelower pad(s). This component thus results only from the weight of thesonde 12.

The sonde is centered in relation to the borehole at the level of thepads while its upper end, fixed to the joint 14, is in the vicinity ofthe wall. The resulting angular offset E between the axis of thecartridge (parallel to the axis of the borehole) and the axis of thesonde body member is then determined by the borehole diameter and thelength of the sonde. For the pads, this angular offset results in anoffset e between the electrodes 25 placed at the mid-length of the padsand the borehole wall, this offset being proportional to the length L ofthe pads designed to be in contact with the wall.

In the present illustrative embodiment, the length L of the pads ischosen to equal about 1.5 D, D being the outer diameter of the sondebody member. With the usual diameters, L corresponds to a length ofabout 15 cm. which furnishes a sufficient bearing surface to prevent thesinking of the pads into the walls. At the same time, this length issufficiently small so that the offset e remains small and does notaffect the quality of the measurements. However, for the case in whichthe upper pad(s) should lose contact, causing the significant weakeningor even the disappearance of the corresponding measurement signals,provision has been made for the possibility of increasing the padbearing pressure.

As was seen in FIG. 2, each pad has an associated leaf spring 29. Thistype of spring urges the pads out towards the bore hole wall andprovides a resilient force which varies little with the extension of thearms. An auxiliary resilient force is furnished by two coil springsmounted in the sonde body member, each of which acts on a pair ofopposite arms. This spring system will be described in detail below.Adjustments to the compression of these springs is controllable from thesurface equipment. Thus it is possible to increase or decrease the padbearing pressure during the measurements, which may be taken eitherintermittently or continuously as the pads are moved along the axis ofthe sonde in accordance with the movement of the cable 11. Bycontrolling the compression of the springs, adequate contact of all thepads with the borehole wall is ensured.

It is thus possible to obtain a very large increase in the force appliedto the pads because the force furnished by the coil springs (verypowerful coil springs can be chosen) has a broad adjustment range. Atthe same time, the resilient force can be limited, for normalconditions, to the value which is simply sufficient for obtaining properpad application. The application of the separate forces of the differentsprings is significant because if a force only suited to extremeconditions were applied constantly, the resistance to the retraction ofthe arms would become very high. In that event, were the pads toencounter a sudden reduction in the size of the borehole, for example ina section following a cave, the pads would "stick" to the walls or therewould at least be a very clear slowing of the movement in relation tothe set speed, this "yoyo" phenomenon being very troublesome. Inaddition, the arm retraction mechanism would have to be more powerful.

A detailed description will not be given to the arm movement and a padbearing pressure control devices. FIGS. 4 and 5 illustrate theconnection of the arms to the sonde body member of axis A-A'. In FIG. 4,only one pad-arm assembly is shown for more clarity. The pad 16 isconnected to the main arm 18 and to the secondary arm 19 by pivots 60and 61 respectively. The pad is also subjected directly to the resilientaction of the leaf spring 29 whose end 62 is engaged in an opening 63provided on the interior side of the pad. The other end of the leafspring is fixed to the sonde body member.

The main arm 18 is connected to the body member 15 by a pivot 65. Asshown in FIG. 5, the main arms have a U section and one of the branchesof the U widens in the direction of the body in the form of an extension66. This extension 66 includes a circular hole in which is fitted a disc67 equipped with a central slot oriented perpendicular to the axis A-A'.The disc 67 is engaged through this slot on a square stud 68 formed onan actuating rod 69 mobile along the axis A-A'. A movement of the rod 69thus causes the pivoting of the main arm 18 around the pivot 65. Thecircle-arc movement of the disc 67 following the movement of the stud 68entails a small lateral movement of the disc. This movement is madepossible by the fact that the disc 67 can slide perpendicular to theaxis A-A' in relation to the stud 68 whose lateral dimension is smallerthan that of the disc slot.

As shown in FIG. 5, the rod 69 has a second stud 70 diametricallyopposite to the stud 68 and which is connected to the main arm 20opposite the arm 18, the mode of connection being the same. The arms 18and 20 are arranged so that their respective extensions 66 are locatedon the opposite sides of the rod 69.

The other pair of main arms 22, 24 is actuated by a second rod 71mounted inside the rod 69 and equipped with two opposite square studs72, 74. As shown clearly in the perspective view of FIG. 7, the rod 69,in its terminal part, has a substantially square section and includes acentral bore 75 in which is slidably mounted the rod 71. The rod 69 alsoincludes a longitudinal opening 76 traversed by the studs 72, 74 of therod 71 and which allows relative movements between the two rods 69 and71.

It is seen in FIG. 5 that the arms 22, 24 are identical to the arms 18,20 and that their connection to the rod 71 is designed exactly like theconnection of the arms 18, 20 to the rod 69. The described arrangementmakes it possible to obtain the coupling of each pair of opposite armsand the independence of the two pairs of arms previously mentioned.

It will be noted that transducers, not shown, are provided for detectingthe movement of each rod 69 and 71. The signals produced by thesetransducers are indicative of the extension of each pair of arms andthus make it possible to determine the size of the borehole along twoperpendicular directions. This information is used, as stated, for dipdetermination.

As shown in FIG. 6, the leaf springs 29 are fixed on a part 77 of thesonde body member by means of screws 78. Reference 79 in FIGS. 5 and 6designates the conductors connecting the electrodes carried by the padsto the electronic cartridge 21.

The actuating rods 69 and 71 can be moved upward (in FIGS. 4 and 6) forthe retraction of the arms. For this purpose a tubular retraction piston85 is provided. Piston 85 is mobile within a case 15a which forms partof the sonde body member, said piston carrying a thrust ring 86 screwedonto its end. As seen in FIGS. 6 and 7, the rod 69 includes a centralpart 87 guided in a part 88 of the sonde body member and, on the endopposite the arms, three sectors of greater diameter distributedregularly around the periphery of the rod 69. Each sector is composed ofa part 89 connected to the central part 87, whose outer diameter issmaller than the inner diameter of the thrust ring 86, a projecting part90 designed for radial contact with the ring 86 to enable the piston 85to move the rod 69, and a terminal part 91 with an outer diametergreater than the inner diameter of the thrust ring 86.

The terminal parts 91 are respectively engaged in the intervals 92provided between fingers 93 constituting the end of a sleeve 94 of thesame outer diameter as the terminal parts 91. The fingers 93 are fixedto respective projecting parts 95 formed on the end of the inneractuating rod 71. These projecting parts 95 have an outer diameter equalto that of the sleeve 94 and hence greater than the inner diameter ofthe thrust ring 86. This permits the retraction piston 85 to also movethe rod 71. It will be noted that the longitudinal dimension of theopening 76 formed in the rod 69 and that of the intervals 92 arecalculated to allow relative movement between the two rods 69 and 71 soas to obtain the independence of the two opposite arm pairs. However, asrequired, the upward movement of the retraction piston 85 causes themovement of the two rods 69, 71 and the retraction of the two pairs ofarms.

The end of the sleeve 94, on the side opposite the arms, isinterpenetrated with the end of a tubular piston 96. The end of thesleeve 94 is made up, as shown in FIG. 7, of three fingers 97 arrangedat 120 degrees and, similarly, the end of the piston 96 is made up ofthree fingers 98, the fingers 97 (or 98) being engaged in the intervalsseparating the fingers 98 (or 97).

Furthermore, a coil spring 100 is mounted around the sleeve 94, and asecond coil spring 101 is placed around the piston 96. The spring 100 ismounted between and bears on the radial projections 102 formedrespectively on the end of the fingers 98 of piston 96 and on theprojecting parts 90 of the external actuating rod 69. The spring 101 ismounted between radial projections 103 formed respectively on the endsof the fingers 97 of the sleeve 94 and a stop collar 104 whichconstitutes the end of the piston 96 on the side opposite the fingers98.

The springs 101, 102 are surrounded by the retraction piston 85, whilepiston 96 and the sleeve 94 surround a tubular part 105 forming part ofthe sonde body member. Inside this tubular part are mounted a hydrauliccontrol unit, not shown in FIG. 6, but described in detail below, andits drive motor. The tubular part 105 is extended by a part 106 oflarger thickness and diameter, the parts 105 and 106 being connected bya radial collar 107. This collar 107 defines a first annular chamber 108with the collar 104 of the portion 96 and a second annular chamber 109with a collar 110 formed on the end of the retraction piston 96 andextending radially inward. The sealing of the chambers 108 and 109 isprovided by seals mounted respectively on the collars 104, 107 and 110.Chambers 108 and 109 communicate with the hydraulic unit throughrespective channels 115, 116.

With the described arrangement, the spring 100 exerts a resilient forceon the external actuating rod 69 since it bears on the projecting parts90 of the rod 69. The spring 101 acts on the internal actuating rod 71since it bears on the projections 103 on this rod 71.

The piston 96 is used for adjusting the biasing force exerted by each ofthe coil springs 100, 101. In the position of FIG. 6, the piston 96 isup against the fixed collar 107 and the springs are under minimumcompression. When the piston 96 moves downward under the action of thepressure created in the chamber 108 by the hydraulic unit, as explainedbelow, the compression of the spring 101 bearing on the collar 104 ofthe piston 96 increases and, as the other spring 100 bears on the radialprojections 102 constituting the end of the piston 96, it is compressedby the same amount as the spring 101. Thus, the increase in theapplication pressure following the movement of the piston 96 is the samefor all the pads.

A displacement transducer, not shown, is housed in the tubular part 105to detect the position of the sleeve 94. This signal produced by thistransducer is thus indicative of the degree of compression of thesprings 100, 101.

The retraction piston 85 is subjected to the action of a coil spring 117which urges it toward the arm extension position (downward in FIG. 6).This spring 117 bears on an annular extension 118 of the fixed part 106.

A tube 120 is fixed inside the fixed part 106 and the extension 118. Theinside of this tube is filled with oil and communicates with a chamber121 which is delimited by a box 122 mounted on the outside of the case15a and by a piston 123. This piston is subjected to tensile force by aspring 124 screwed on one end to the part on the piston 123 and on theother end to a stop piece 125 fixed on the other end of the tube 120.The extension 118 and the piston 123 define between them a chamber 126which communicates with the exterior. During a measurement operation ina borehole, the drilling mud penetrates into the chamber 126 and the mudpressure is imparted by the piston 124 to the oil present in the chamber121 and to the inside of the tube 120, thereby providing suitablepressure balancing. This oil flows in the hydraulic unit and forms thereservoir necessary for its operation.

The hydraulic unit is shown schematically in FIG. 8. To betterunderstand its relationship with the elements described above, aschematic representation is given in FIG. 8 of the piston 85 controllingthe retraction of the arms. The piston 85 is subjected to the pressureprevailing in the chamber 109, hereinafter called the retractionchamber, and to the resilient action of the spring 117. Also shownschematically is the piston 96 designed to adjust the compression of thesprings 100 and 101, this piston being subjected to the pressure in thechamber 108, hereinafter called the compression chamber.

The hydraulic unit includes a distributor, designated generally by thereference 130, which is made up of a fixed cylinder 131 and a mobilepiston 132 of axis B-B'. The cylinder 131 includes, associated with eachof its terminal walls 133, 134, a transverse partition 135, 136 equippedwith a central hole for the passage of the piston 132, seals beingprovided for tightness. The chamber 137 defined between the wall 133 andthe partition 135 communicates at all times with the retraction chamber109 via the channel 116 and can communicate with the reservoir via acircular opening 139 centered on the axis B-B' of the piston 132. InFIG. 8, the reservoir is designated by the reference 140.

Symmetrically, on the other end of the cylinder 131, the terminal wall134 and the partition 136 define a chamber 141 which communicates at alltimes with the compression chamber 108 via the channel 115 and which cancommunicate with the reservoir via a circular opening 143 centered onthe axis of the piston 132. Relief valves 145, 146 are branchedrespectively between the channels 115 and 116 and the reservoir.

The piston 132 has a generally tubular structure with a portion 150going through the partition 135 and a portion 151 going through thepartition 136. At the connection of these two portions is a transversepartition 152 which separates in a sealed manner the space between thepartitions 135 and 136 of the cylinder into two chambers, namely thechamber 154 between the fixed partition 135 and the mobile partition152, and the chamber 155 between the mobile partition 152 and the fixedpartition 136. The chamber 155 communicates with the inside of theportion 150 of the piston through an opening 157. The chamber 154 can beconnected by a channel 160 to the reservoir by means of a normally openelectromagnetic valve 161 and a ball valve 162. A narrow orifice 159 isprovided in the seat of the ball valve for a purpose which will beexplained below.

The chamber 155 is connected via a channel 163 to a pump 164 driven by amotor 165, a nonreturn valve 166 being mounted in this channel. The pump164 draws oil from the reservoir through a filter 167. The chamber 155can also be placed in communication with the reservoir via a channel 168in which is provided a normally closed electromagnetic valve 169.

The piston 132 is urged toward the right (in FIG. 8) by a coil spring170 mounted between the fixed partition 135 and the partition 152 of thepiston.

A valve member 171 is mounted inside the portion 151 of the piston 132(located on the right in FIG. 8). This valve member includes a cone 172formed on the end of a shank 173 and, on the other end, a truncatedflare 174 joining the shank. The diameter of the shank 173 is slightlylarger than that of the opening 143 so that the cone 172 is capable ofclosing off the opening 143 as shown in FIG. 8. A coil spring 175bearing on an inner shoulder 176 urges the valve member 171 into theclosed position (toward the right in FIG. 8). As seen in FIGS. 8 and 9,the truncated flare 174 is designed to cooperate with a seat made up ofthe edge of another inner shoulder 177 near the end of the piston,thereby closing the communication between the chamber 141 and the insideof the piston 132 connected to the chamber 155. It should be noted inthis regard that, when the piston 132 is up against the wall 134, asshown in FIG. 8, and the cone 172 closes off the opening 143, thetruncated flare 174 is not in contact with its seat. In other words, asshown better by the detail view of FIG. 9, the axial distance a betweenthe edge 180 of the shoulder 177 designed to be in contact with thetruncated flare 174 and the end face of the piston 132 which comes upagainst the wall 134 is smaller than the axial distance b between thecircular zone 181 of the truncated flare 174 which comes into contactwith the edge 180 and the circular zone 182 of the cone 172 which comesin contact with the opening 143.

The end of the piston 132 which is in contact with the terminal wall134, in the position of FIG. 8, includes slots 178 which allowcommunication between the inside of the portion 151 of the piston andthe chamber 141 when the piston 132 is in contact with the wall 134.

A symmetrical arrangement is provided on the other end of the piston 132with a valve member 191 of the same form as the valve member 171,equipped with a cone 192 to close off the opening 139 and a truncatedflare 194 designed to cooperate with an inner shoulder 197 to close offthe communication between the chamber 137 and the inside of the tubularportion 150 of the piston. The valve member 191 is urged toward theclosed position by a coil spring 195 bearing on an inner shoulder 196.

In the position shown, the configuration of the valve member 191 is theopposite of that of the valve member 171, i.e. it is in contact with theshoulder 197 and does not close off the opening 139. Moreover, a ball200 is confined between inner projections 196 allowing the passage ofoil and an inner shoulder 201 of the portion 150, serving as a seat forthe ball 200. A small orifice 202 is provided in the seat 201 tomaintain communication between the inside of the portion 150 and theinside of the portion 151 when the ball 200 is against its seat.

The operation of the hydraulic unit will now be described during adipmetering operation.

The position shown in FIG. 8 is the rest position, the arms beingextended under the action of their springs. The piston 132, under theaction of the spring 170, is up against the wall 134. The motor 165 isstopped. The electromagnetic valves 161 and 169, the electromagnets ofwhich are not energized, are closed. The retraction chamber 109communicates with the reservoir, while the valve member 171 allowscommunication between the compression chamber 108 and the chamber 155.The chamber 154 is closed by the electromagnetic valve 161.

The first operation consists in placing all the chambers in pressureequilibrium with the reservoir. To accomplish this, the opening of thevalves 161 and 169 is actuated. The compression chamber 108 is thusplaced in communication with the reservoir via the chamber 155 and thevalve 169. Likewise, the chamber 154 is placed in communication with thereservoir via the valve 161. The retraction chamber 109 is already incommunication with the reservoir. Once this equilibrium is reached, theelectromagnets of the valves 161 and 169 are no longer energized.

It is then necessary to move the piston 85 to retract the arms forlowering to the borehole. For this, the motor 165 is started up to drivethe pump 164, and the valve 161 is opened to place the chamber 154 incommunication with the reservoir so that the oil present in this chamberdoes not oppose the movement of the piston 132. The increase in pressurein the chamber 155 due to the operation of the pump causes a movement ofthe piston 132 against the action of the spring 170 until it comes upagainst the terminal wall 133. Owing to this movement, the cone 192 ofthe valve member 191 closes off the opening 139, thereby closing thecommunication of the retraction chamber 109 with the reservoir.Meanwhile, the truncated flare 194 is moved away from its seat, placingthe retraction chamber in communication with the chamber 155 via theinside of the portion 150 of the piston.

The reverse process takes place on the other end of the piston 132. Thetruncated flare 174 is placed in contact with its seat while the cone172 stops closing off the opening 143. The compression chamber 108 thusstops communicating with the chamber 155 but is placed in communicationwith the reservoir. The result is that the pressure increase in theretraction chamber moves the piston 85 against the action of the spring117, and the arms are retracted along the sonde body member. As thecompression chamber is at the pressure of the reservoir, the piston 96remains in the rest position.

Once the arms have been retracted, the motor 165 is stopped. The returnof the valve 161 to the closed position can take place before thestopping of the motor. After the stopping of the motor, the valves 161and 169 having been closed, no movement of the piston 132 takes placeand it remains up against the wall 133. The arms hence remain retracted.

The lowering of the apparatus into the borehole can thus be carried out.When the apparatus has reached the depth at which the measurementmovement is to begin, the arms must be extended so that the pads comeinto contact with the borehole wall. To accomplish this, the two valves161 and 169 are opened. The piston 132 begins to move under the actionof the spring 170 because the pressure drops in the chamber 155.

As there is a distance (b-a) which must be traversed by valve member 191before truncated flare 194 engages seat 197 as previously described, thevalve member 191 remains in the same position at the beginning of pistonmovement. The retraction chamber does not yet communicate with thereservoir. It no longer communicates with the chamber 155 except throughthe small orifice 202 because the pressurized oil it contains appliesthe ball 200 against its seat 201. This prevents the pressurized oilfrom going through the valve 169 and damaging it.

The return of the piston 132 and the valve member 191 into the positionof FIG. 8, establishes the communication of the retraction chamber withthe reservoir and the closing of its communication with the chamber 155.The valves 161 and 169 are then closed.

During measurement, if it is wished to increase the pad bearingpressure, the piston 96 must be moved. For this, the motor 165 isstarted up, the valves 161, 169 remaining closed. The pressure increasesin the chamber 155 and hence the compression chamber 108 whichcommunicates with it.

The relief valve 145 defines the maximum valve P_(max) that the pressurecan reach in the compression chamber. To set the pressure at a valuelower than P_(max), the motor need only be stopped when this value isreached. The pressure keeps this value after the motor is stopped. Ifafterward it is considered that the pad bearing pressure can come backto a lower value, the pressure in the compression chamber is reduced.For this, the valve 169 is opened to place the compression chamber incommunication with the reservoir via the chamber 155. The valve 169 isclosed when the desired value is reached.

To bring the pressure in the compression chamber back to its minimumvalue, namely the pressure of the reservoir, there are twopossibilities. The valve 169 can be opened, in which case the drop inpressure is slow, the orifice offered by this valve being small. Theother possibility, allowing a faster pressure drop, consists in openingthe two valves 161 and 169. The chamber 154 is in communication with thereservoir and because of the difference in the pressure between thechamber 154 and the chamber 155, the piston 132 moves against the actionof its spring, moving the valve member 171. The compression chamber 108is thus placed in communication with the reservoir through the opening143, thus allowing the pressure in this chamber to drop rapidly.

At the end of the measurement movement, the arms are retracted using theprocedure described above.

An emergency procedure moreover exists for obtaining rapid retraction ofthe arms when a high pressure prevails in the compression chamber 108.For this purpose, the valve 161 is opened. The oil which flows from thechamber 154 applies the ball 162 against its seat and therefore the oilcan flow only through the narrow orifice 159 provided in this seat. Thepiston 132 then moves toward the left because the pressure is high inthe chamber 155. However, the speed of piston 132 is limited by thespeed at which the oil can escape through the orifice 159.

The motor 165 is then started up. The movement of the piston 132 hasclosed the communication beteen the retraction chamber 109 and thereservoir, and has established communication between the retractionchamber and the chamber 155, on the one hand, and between thecompression chamber 108 and the reservoir on the other. The pressurethus increases in the retraction chamber and the piston 85 moves toretract the arms.

The apparatus described is adapted to measurements in inclinedboreholes. In vertical (or at least only slightly inclined) boreholes,the articulated connection between the sonde and the cartridge is nolonger necessary because the radial component of the weight isnegligible. In this case, the sonde can be fixed rigidly to thecartridge and centering devices can be mounted on the sonde or thecartridge.

The described arrangement which makes it possible to increase the padpressure is also advantageous in a vertical borehole, particularly inthe presence of a high-viscosity drilling mud.

It will be understood by those skilled in the art that theabove-described embodiment of the invention is intended to be merelyexemplary, and that it is susceptible of modification and variationwithout departing from the spirit and the scope of the invention.

We claim:
 1. Method for logging in a section of a deviated borehole,comprising the following operations:lowering into the borehole down to agiven depth, by means of a cable, an apparatus including an elongatedmeasurement sonde and an elongated cartridge connecting the sonde to thecable, the sonde being equipped with four measuring pads which carrymeasurement means and which are distributed regularly around the sonde,the pads being placed at the ends of respective arms hinged on thesonde, the opposite pads being kept symmetrical in relation to the axisof the sonde, the dimension of the pads parallel to this axis being atmost approximately equal to twice the transverse dimension of the sonde;applying resiliently the pads against the borehole wall while keepingthem parallel to the axis of the sonde, the sonde being centered inrelation to the borehole at the height of the pads; moving the padsalong the borehole by pulling on the cable while allowing an angularoffset between the axis of the cartridge and that of the sonde, wherebythe cartridge can rest on the borehole wall.
 2. Method according toclaim 1, wherein during said moving step, signals are produced which areindicative of the inclination of the sonde axis in relation to thevertical and its orientation in relation to a reference azimuth. 3.Method according to either of claims 1 or 2, wherein the resilient forceapplying the pads against the borehole wall is adjustable during theirmovement.
 4. Method according to claim 3, wherein said resilient forceresults from the addition of a first force which is substantiallyconstant whatever the position of the pads and a second adjustableforce.
 5. Logging apparatus for investigating the formations traversedby a section of a deviated borehole, comprising: an elongated electroniccartridge arranged to be connected to a cable; and an elongatedmeasuring sonde connected to the cartridge by means of a joint allowingan angular offset between the axis of the sonde and that of thecartridge, the sonde including a body member, four main arms articulatedon the body member and distributed regularly around said body member,the opposite arms being forced to remain symmetrically in relation tothe axis of the sonde, a secondary arm associated with each main arm,articulated on the body member, four measuring pads connected to theends of the respective main arms and secondary arms in a parallelogramconfiguration, the pads thus remaining parallel to the axis of thesonde, the dimension of the pads parallel to the axis of the sonde beingat most approximately equal to twice the transverse dimension D of thesonde, resilient means acting to extend the pads away from the bodymember and a mechanism capable of overcoming the action of the resilientmeans to retract the pads against the body member.
 6. Apparatusaccording to claim 5, wherein said dimension of the pads issubstantially equal to 1.5 times the outer diameter of said elongatedsonde.
 7. Apparatus according to either of claims 5 or 6, wherein thesonde includes means for producing signals indicative of the inclinationof its axis with respect to the vertical and signals indicative of theorientation of said axis with respect to a reference azimuth. 8.Apparatus according to claim 5, wherein said resilient means includesmeans for applying an adjustable resilient force to the pads.